According to “Jack the Giant­ Slayer” director Bryan Singer, newcomer Eleanor Tomlinson was cast as feisty Princess Isabelle “because she had the best chemistry” with Jack (Nicholas Hoult of “Warm Bodies”).

“And fortunately,” he added­ of his 5-foot-10 leading lady, “she’s tall, because he’s gigantic.”

“Isabelle is a princess who doesn’t want to be,” Tomlinson, 20, said of her introduction to American movie­goers.

“Isabelle,” she added at the Hampton Court Castle where “Jack” filmed, “is not in a tower waiting for her handsome prince to rescue her. She’s very much a people’s princess. She wants to experience real life.”

So the two meet when farm boy Jack rescues Isabelle from an unruly bunch at the fair she’s attending in disguise.

“He doesn’t realize who she is; he’s just doing the noble thing, even though he’s a commoner, and that’s kind of where it starts off.”

As for screen chemistry, “I think that’s what holds a movie like this together,” she said. “Because at the core is a relationship between two people that carries one guy up a beanstalk to save this girl that he hardly knows.

“You take away the giants, you take away the CGI, if that relationship isn’t be­lievable, you don’t have a story.”

The daughter of actor Malcolm Tomlinson, Tomlinson got her break at 13 playing the young Jessica Biel in “The Illusionist.”

“I went to an all-girls school in a small town, and that comes with its own challenge. When you’re the different one, you become a target. I was bullied a lot at school.

“But coming from a strong family background, I knew that I was doing what I wanted­ to do.

“I would just put out a message to kids, ‘Don’t let anyone tell you that you can’t be that person.’ I remember people going up to my dad — ‘How dare you put her into the industry this young? She should be out playing.’ That’s just not me. Not everyone is the same.

“Acting’s just been a passion of mine since I was very small and it always will be.”